1. Field of the Technology
The present technology relates to an image forming apparatus.
2. Description of the Related Art
The adoption of electrophotographic image forming apparatuses as copying machines, printers, facsimile machines, and the like equipment has been widespread. In general, image formation is carried out by an electrophotographic image forming apparatus as follows. As a photoreceptor, there is used a photoreceptor on which surface a photosensitive layer containing a photoconductive substance is formed. After electric charge is applied to the surface of the photoreceptor in such a way that the entire surface is charged evenly, an electrostatic latent image corresponding to specific image data is formed thereon through various process steps for image formation. The electrostatic latent image is developed into a toner image by using a developer containing toner that has been supplied from a developing section. The toner image is directly transferred onto a recording medium such as a paper sheet, or is transferred onto an intermediate transfer medium once and is thereafter transferred onto a recording medium. Lastly, the toner image transferred onto the recording medium is fixed into place. The fixation of the toner image onto the recording medium can usually be achieved by heating and pressing the recording medium with use of a fixing section constructed based on a heat fixing method, for example, a fixing roller comprising a heating section.
However, there has been a growing trend recently to achieve energy conservation as a countermeasure against global warming. As a natural consequence thereof, a reduction in power consumption required to fix a toner image onto a recording medium has come to be increasingly demanded of an electrophotographic image forming apparatus. Unfortunately, the heat fixing method presents the following disadvantages. Firstly, a heating section is used inside the image forming apparatus, wherefore the apparatus is interiorly heated to a high temperature. This creates the need to enhance the heat resistance of constituent components, which results in an undesirable increase in material costs. Secondly, image fixation cannot be effected until a part to be fixed has been heated to a predetermined temperature. In this case, the time required for the to-be-fixed part to reach the predetermined temperature, namely, warm-up time, tends to be increased. Thirdly, much time needs to be spent in fixing a multi-color toner image onto a recording medium as compared to the case of fixing a monochromatic toner image. Accordingly, a reduction in time required to fix a multi-color toner image has been sought after.
In view of the foregoing requirements, a wet fixing method has been known that employs a fixing fluid containing water and a liquid which is dissoluble or dispersible in water and produces the effect of softening or swelling out toner. According to the wet fixing method, a toner image is brought into a softened or swollen state through the application of the fixing fluid. In this state, the toner image is adhered to a recording medium, and is then fixed into place by pressurization. The wet fixing method consumes far less power than does the heat fixing method, and is thus of a useful method from energy-saving standpoint. Another advantage is that the wet fixing method does not require a large amount of heat to fix a multi-color toner image. This, in contrast to the case of adopting the heat fixing method, enables a reduction in fixation time. Accordingly, various proposals have been made for further improvements of the wet fixing method.
For example, there is a proposal of an image forming apparatus that, when applying a fixing fluid to a toner image transferred onto a recording medium and fixing the toner image to the recording medium, heats or pressurizes the toner image before applying the fixing fluid, thereby increasing toner-to-toner binding force of toner forming the toner image (refer to Japanese Unexamined Patent Publication JP-A 2004-294847, for example). According to the image forming apparatus of JP-A 2004-294847, since the toner forming the toner image is softened to some extent by heating or pressing before application of the fixing fluid, it is possible to reduce the amount of the used fixing fluid. On the other hand, however, in a case where recording paper, which is the most generally used as a recording medium, is used, the recording paper absorbs the fixing fluid in large amounts. Therefore, substantial reduction of the amount of the consumed fixing fluid is not realized as a whole. Accordingly, the following method has been proposed in a lot of documents. That is, the method comprises steps of: applying a fixing fluid to a toner image on an intermediate transfer belt hardly absorbing the fixing fluid; softening and/or swelling toner forming the toner image on the intermediate transfer belt; making the intermediate transfer belt and a recording medium in contact with each other in a transferring and fixing section; and transferring and fixing the toner image on the intermediate transfer belt to the recording medium by adhesive force of the softened and/or swelled toner. According to this method, since the intermediate transfer belt does not absorb the fixing fluid, it is possible to substantially reduce the amount of the consumed fixing fluid, whereas a problem resulting from use of the intermediate transfer belt newly arises. In this method, the intermediate transfer belt and the recording medium are made in contact with each other in the transferring and fixing section, the toner image on the intermediate transfer belt is transferred and fixed to the recording medium, and thereafter, the recording medium is peeled from the intermediate transfer belt. However, depending on timing of peeling the recording medium from the intermediate transfer belt, the toner image may be transferred to the recording medium insufficiently, thereby remaining on the intermediate transfer belt. In this case, image defect is caused in a formed image.
According to research by the inventor, what determines whether a toner image is transferred to a recording medium or remains on the intermediate transfer belt is presumed to be, mainly, the amount of the applied fixing fluid, and fluctuations of adhesive force between the intermediate transfer belt and swelled and/or softened toner (referred to as the “adhesive force A” hereafter) and adhesive force between the recording medium and the swelled and/or softened toner (referred to as the “adhesive force B” hereafter) during a required time from when the fixing fluid is applied to the toner image on the intermediate transfer belt to when the recording medium is peeled from the intermediate transfer belt.
FIGS. 20 to 22 are graphs each illustrating a relation between a time after application of the fixing fluid to the toner image and the adhesive force (the adhesive force A and the adhesive force B). In the drawings, the time after application of the fixing fluid to the toner image is plotted on the horizontal axis, and is referred to as the “time after application.” The adhesive force is plotted on the vertical axis. The adhesive force A is shown by a solid line, and the adhesive force B is shown by a dashed line. The adhesive force is a value measured by a measurement method described later. FIG. 20 is a graph illustrating a relation between the time after application and the adhesive force in a case where the amount of the applied fixing fluid is more than an amount to sufficiently swell and/or soften the toner image. Since the intermediate transfer belt retains heat or static electricity transmitted when coming in contact with other components, the adhesive force A is somewhat high already at a point that application of the fixing fluid starts. When application of the fixing fluid starts in this state, the adhesive force A gradually increases from a value at the point that application starts because the toner forming the toner image is swelled and/or softened. After that, because the fixing fluid is dried up, and releasability of the surface of the intermediate transfer belt appears, the adhesive force A gradually decreases, and becomes constant. Although the adhesive force B is considerably smaller than the adhesive force A at the point that application of the fixing fluid starts, the adhesive force B increases more rapidly than the adhesive force A when application of the fixing fluid starts. The maximum value is determined based on surface roughness of the recording medium, permeability to the recording medium, and so on, and the adhesive force B becomes constant. In a case where the amount of the applied fixing fluid is large, the maximum value of the adhesive force A is larger than the maximum value of the adhesive force B.
FIG. 21 is a graph illustrating a relation between the time after application and the adhesive force in a case where the amount of the applied fixing fluid is an amount that is not too much nor too little to swell and/or soften the toner image (a proper amount). In this case, since the amount of the applied fixing fluid is proper, the fixing fluid hardly penetrates through an interface between the intermediate transfer belt and the toner image. Therefore, the adhesive force A keeps an initial value resulting from the heat or static electricity retained by the intermediate transfer belt regardless of a lapse of time, and the value is constant. Although the adhesive force B rapidly increases right after application of the fixing fluid because the fixing fluid penetrates through the toner as in the case of FIG. 20, the adhesive force B becomes constant when reaching the maximum value. In a case where the amount of the applied fixing fluid is proper, the maximum value of the adhesive force A is smaller than the maximum value of the adhesive force B. FIG. 22 is a graph illustrating a relation between the time after application and the adhesive force in a case where the amount of the applied fixing fluid is less than the amount to sufficiently swell and/or soften the toner image. In this case, since the amount of the applied fixing fluid is small, the fixing fluid hardly penetrates through the interface between the intermediate transfer belt and the toner image as in the case of FIG. 21. Therefore, the adhesive force A keeps the initial value resulting from the heat or static electricity retained by the intermediate transfer belt regardless of a lapse of time, and the value is constant. The adhesive force B rapidly increases right after application of the fixing fluid because the fixing fluid penetrates through the toner. However, since the amount of the applied fixing fluid is small, the adhesive force B rapidly increases temporarily, and becomes constant at a value lower than the adhesive force A.
For the relations as described above, in order to peel the recording medium from the intermediate transfer belt in a state where the toner image is transferred to the recording medium in the almost complete form, it is necessary to select a period of time in which the adhesive force B is higher in value than the adhesive force A in the time after application. Consequently, it is possible to obtain a high-quality image without image defect. On the other hand, in toner images formed by a color image forming apparatus, monochrome toner images and laminated toner images formed by laminating toner images of two different colors or more are mixed. When a proper amount of fixing fluid to swell and/or soften the monochrome toner image is applied to the laminated toner image, the amount of the applied fixing fluid becomes insufficient, and the adhesive force B becomes lower than the adhesive force A at all times as illustrated in FIG. 22. Therefore, there is a high possibility that the toner image remains on the intermediate transfer belt. On the contrary, when a proper amount of fixing fluid to swell and/or soften the multicolor toner image is applied to the monochrome toner image, the amount of the applied fixing fluid becomes excessive, and it is necessary to design so as to peel between time t1 and time t2 or after time t3 as illustrated in FIG. 20. Therefore, such an image forming apparatus is desired that the adhesive force B is higher in value than the adhesive force A at all times and a recording medium can be peeled from the intermediate transfer belt in a state where a toner image is almost completely transferred onto the recording medium.